How Amazon's Camu-Camu Fruit Produces Vitamin C Unveiled

David Palenski
16th January, 2024

How Amazon's Camu-Camu Fruit Produces Vitamin C Unveiled
Photo adapted from: Seig / CC BY (Source)
Vitamin C, also known as L-ascorbic acid, is vital for both human and plant health. While we obtain it through our diet, plants create it themselves through complex biochemical pathways. Understanding these pathways is important for improving crop yields and potentially increasing vitamin C content in foods. Researchers at the University of São Paulo[1] have recently investigated the detailed steps of vitamin C production in the camu-camu plant ( Myrciaria dubia), a fruit exceptionally rich in this nutrient. The camu-camu plant utilizes a primary pathway for vitamin C creation called the Smirnoff-Wheeler pathway. This pathway involves a series of six key enzymes, each performing a specific chemical transformation. The study focused on three of these enzymes: GDP-d-mannose 3',5'-epimerase (GME), L-galactose dehydrogenase (L-GalDH), and L-galactono-1,4-lactone dehydrogenase (L-GalLDH). The goal was to determine the precise three-dimensional structure of each enzyme, which provides clues about how they function. Determining the structure of these enzymes is challenging, but crucial. Enzymes are proteins, and their shape dictates their activity. By understanding the shape, scientists can understand how the enzyme interacts with the molecules it works on (its substrates) and any helper molecules (cofactors) it needs. This knowledge can then be used to potentially modify the enzymes to improve their efficiency or alter the vitamin C production process. The researchers used a technique called X-ray crystallography to visualize the enzymes. This involves creating crystals of the enzyme and then bombarding them with X-rays. The way the X-rays scatter reveals the arrangement of atoms within the crystal, allowing scientists to build a detailed 3D model. This process, while powerful, can be complex and requires sophisticated software for data analysis[2]. The study revealed several important details. The structure of GME showed the substrate (GDP-d-mannose) bound in a slightly distorted shape within the enzyme’s active site – the region where the chemical reaction takes place. This distortion is thought to be important for the enzyme’s catalytic mechanism, the way it speeds up the chemical reaction. GME is known to convert GDP-d-mannose to GDP-l-galactose, a crucial step in vitamin C synthesis, and is also involved in bacterial lipopolysaccharide synthesis[3]. The structure of L-GalDH showed a surprising difference compared to a similar enzyme found in spinach. Specifically, the way the molecule NAD+ (a cofactor) binds to the enzyme and affects the surrounding loop structure differs between the two plant species. This suggests that the precise mechanism of L-GalDH may vary between plants. Finally, the study provided the first-ever structural description of L-GalLDH from camu-camu. This allowed the researchers to confirm the roles of specific amino acids (the building blocks of proteins) that are known to be important for the enzyme’s activity, including those involved in binding to FAD, another essential cofactor[4]. L-GalLDH, along with L-gulono-1,4-lactone oxidase, are involved in the terminal steps of AsA biosynthesis[4]. These findings build upon previous research identifying multiple biosynthetic pathways for vitamin C in plants, utilizing different starting molecules like D-mannose, L-galactose, D-galacturonate, and myo-inositol[4]. The camu-camu plant appears to rely heavily on the L-galactose pathway, as evidenced by studies showing a strong correlation between AsA levels and the expression of genes involved in this pathway, particularly GDP-L-galactose guanyltransferase and GDP-mannose-3',5'-epimerase[5]. In fact, increasing the levels of GDP-L-galactose guanyltransferase in other plants, like Arabidopsis, has been shown to significantly boost vitamin C content[5]. The detailed structural information obtained in this study provides a foundation for future research aimed at enhancing vitamin C production in plants. This could have significant implications for improving the nutritional value of crops and developing new biotechnological applications.

FruitsNutritionBiochem

References

Main Study

1) Structural insights into the Smirnoff-Wheeler pathway for vitamin C production in the Amazon fruit Camu-Camu.

Published 15th January, 2024

https://doi.org/10.1093/jxb/erae016

Related Studies

2) PHENIX: a comprehensive Python-based system for macromolecular structure solution.

https://doi.org/10.1107/S0907444909052925

3) GDP-Mannose 3,5-Epimerase: A View on Structure, Mechanism, and Industrial Potential.

https://doi.org/10.3389/fmolb.2021.784142

4) Recent progress on the characterization of aldonolactone oxidoreductases.

https://doi.org/10.1016/j.plaphy.2015.11.017

5) Gene expression studies in kiwifruit and gene over-expression in Arabidopsis indicates that GDP-L-galactose guanyltransferase is a major control point of vitamin C biosynthesis.

https://doi.org/10.1093/jxb/ern327


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